The present invention relates to a non-aqueous electrolyte secondary battery that keeps the favorable cell performances and has the enhanced high-temperature storage properties and high energy density.
Non-aqueous electrolyte secondary batteries using an alkali metal, such as lithium or sodium, as a negative electrode or an anode are intensively studied since they have a high electromotive force and are expected to have a higher energy density than conventional nickel-cadmium storage batteries and lead-acid storage batteries. Especially the non-aqueous electrolyte secondary batteries using Li as the anode have been practically applied for power sources of portable cordless apparatuses, such as information and communication apparatuses and audio-visual apparatuses, and mass-produced.
The practical non-aqueous electrolyte secondary batteries use a carbon material as the anode and LiCoO.sub.2 as a positive electrode or a cathode. Studies and researches are concentrated on the materials of the cathode and the anode, in order to further reduce the cost and enhance the energy density.
Development of new materials which are manufactured at low cost and have high performances will expand the application of the non-aqueous electrolyte secondary batteries to a variety of fields, such as electric vehicles, as well as the current small-sized portable apparatuses.
The allowable temperature range of commercially available non-aqueous electrolyte secondary batteries is -20.degree. C. to 60.degree. C. Under the conditions of conventional applications, the batteries are used at or around room temperature, so that no significant problems have arisen with respect to the temperature. The future expansion of their applications may, however, cause the batteries to be used in severer conditions.
A lithium secondary battery, which is a currently available non-aqueous electrolyte secondary battery, internally produces heat during discharge. The small-sized battery produces only a small quantity of heat and has favorable radiation of heat from the surface of the battery. The large-sized battery used, for example, for electric vehicles to generate a large amount of electricity, on the other hand, produces a greater quantity of heat than the quantity of radiation and may temporarily has a high internal temperature. The battery is also exposed to high temperatures when being applied to apparatuses which produce heat.
In case that the lithium secondary battery under the charged condition is exposed to high temperatures, the properties of the battery drastically deteriorate due to the expected reaction of an active material in the charged state with a non-aqueous electrolyte. The once deteriorating battery due to the high temperatures does not recover its properties.
It is accordingly required to prevent deterioration of the properties due to the heat when the non-aqueous electrolyte secondary battery is applied to the conditions of high temperatures.